What is the difference between the imperfect and the preterite in Spanish?

The imperfect tense in Spanish is used to refer to an action that was ongoing in the past. For example, you would use the imperfect to say something like: Yo iba a tu casa cada semana. Note that this was an action happening habitually in the past, and therefore spread out over a certain period of time. In contrast, the preterite is used to express an action which is instant and has happened at a specific moment. For instance:Ayer yo vi a Maria. I saw Maria yesterday. This is an action that happened at a specific time (being yesterday) and once-it is not described as a regular, ongoing action. Additionally, it is very instant.